In forming small parts of metal powder it has been customary to mix the powder with a small quantity of binder material and compact it in a mold with sufficient pressure to enable the compacted material to hold its shape after withdrawal from the mold. The shaped material is then sintered in an oven to drive off the binder material and to impact cohesiveness to the shaped mass. This method of forming is applied not only to powders composed entirely of metal particles but also to powders which are formed into refractory shapes referred to as cermets, such as cemented tungsten carbides.
While compaction of the powders is usually achieved in a press having reciprocating movement, it may be better achieved in the sense of enhancing uniformity by the application of centrifugal force as illustrated in Wolff et al. U.S. Pat. No. 2,304,723 or a combination of vibration and centrifugal force as is shown in Lester, U.S. Pat. No. 2,435,227. The Wolff technique overcomes a prior art disadvantage in that it uses a carbon, graphite or metal mold to avoid breakage of the article. Sintering a plug in such a mold causes the article to shrink, whereupon it slips easily out of the mold. It is suggested also to use low-melting lead as a mold, because it will melt away from the sintered article. Lester, on the other hand, compresses the loose powder into a heat resistant mold comprising graphite, aluminum oxide or metal. One obtains a uniform preform by this technique but removal from the mold can cause breakage, as noted by Wolff. Murray, U.S. Pat. No. 3,502,755 describes making fired or sintered articles, having indentations, from metal powders or ceramics. In this technique, the powder is placed in a rubber bag mold and the mold is squeezed by pressure applied to the outside of the bag. To produce an indented shape, Murray provides a cup-shaped, thin rubber dam within the powder body and, after releasing the pressure, the dam is removed, pulling out a plug of waste material corresponding to the shape of the desired indentation (and the dam). Murray suggests that one way of removing the dam is to burn it out during the firing operation. However, the use of a bag molding technique cannot provide the accurate dimensions required by the designer because, as Murray observes, only relatively simple shapes such as rods and cylinders can be produced, or tubes, if a mandrel is inserted in the powder mixture and removed later.
The present invention is directed toward a process for making shaped articles with close tolerances using centrifugal compaction into a destructible thin walled polymeric mold which is preferably non-compressible, and which process does not use a conventional graphite or metal mold except as a support. Whereas the molds described in the prior art teachings of Lester and Wolff react with carbide powders during sintering, in the present invention, the thin walled polymeric mold is not compressible in a metal support and holds the accurate dimensions required by the designers. In addition, the polymeric mold, unlike the prior art rubber bag mold, has good lubricity. It is therefore, easily removed from a metal support mold. On the other hand, the prior art rubber mold cannot be readily removed from a metal support without possibly rupturing or breaking the compressed shaped compact within the mold.
Another advantage of the process of the present invention is that the polymeric mold is disposable during sintering without harmful effect. The polymeric mold is removed completely and cleanly in the furnace, similar to the elimination of paraffin in a powder mix.
In addition, centrifugal compaction into a disposable, destructible mold in accordance with this invention provides the designer with greater freedom in designing parts. For example, a tungsten carbide part for which there is considerable demand consists of a cylinder terminating at one end in a generally cone-shaped portion. This part can be made by press compaction only if the ratio of the cone height to the total height is less than 51%, and the cone angle is greater than 19.degree.. Obviously, it cannot be made by Murray's bag molding technique. These limitations are not present if the present invention is used.